Engine Oils and Lubrication
By: John Gislason
This is a Technical Article for CrownVic.Net. My primary source for this article was
“High-Performance Automotive Fuels & Fluids” by Jeff Hartman. Along with a good dose of conversation with
Clint Ward, Lubrication Engineer for “Lubetech”, here in
To Begin, there are three distinct types of oil commonly
available, Conventional (Dinosaur), Semi-Synthetic, and Full Synthetic. This article will focus on “Dino” oil, with
brief notes on Full Synthetics.
Motor Oil
Is not just “oil”, each quart is made up of components, the
largest two are Oil, and Viscosity Improvers “
Oil “Weight”
is generally considered to be it’s “thickness”, and that is
true of the base stock oil, but untrue of viscosity improved oils. An oil’s weight, is determined by the “SAE“
test regimines, and is represented in multi weight oils by the “W” number, and
is fixed for that base oil. All of the
VI in the world, will not change the weight of the oil, or improve it’s
lubricating abilities, it will only modify it’s viscosity.
Cold start
is considered to be core engine temperature at or colder
than ambient, this is when the greatest wear occurs. It is generally accepted, that approximately
80% of ALL engine wear occurs on a cold start due to lack of oil pressure, and
the resulting lack of lubrication. It is
worth pointing out, that 0W, 5W, 10W, and 15W engine oils, all have viscosities
very close, but at different below zero temps.
For 5W the “equal viscosity” occurs at a temp. of -25 deg. C, and for
15W is at -15 deg. C. The relationship
for these four weights is linear, varying directly with temperature in degrees
Celsius. This also has implications for
cold pumpability. The cut-off for cold
pumping, falls 5 degrees C colder, than the “equal viscosity” temperature.
So, for 5W oil at -25 C, the
borderline pumping temp would be -30 deg. C, and so on. The “W” does stand for “winter” weight, it is
also the “base stock weight” for multi viscosity oils.
Hot operating
conditions, in multi-viscosity oils are dealt with using
Viscosity Improvers “VIs”. These
molecules can improve viscosity by two methods.
One, they can bond with each other into larger strings of
molecules. Two, they can “swell” by
themselves to thicken the oil. The SAE
standards for “hot” oils are set at 100 deg C (212 F) oil temperature. This oil rating is referred to as the “summer
weight”. 15W oil and 15 wt. oil are the
same thing, however 5W -20 and 20 wt represent two different things. The 5W-20 oil IS 5 wt oil with VI to augment
it’s hot performance, while 20 wt oil, IS 20 weight oil without any VI. The protection available to the engine
components is a function of base stock oil weight (and quality), and oil
temperature (because it directly affects the viscosity of the base stock oil).
Viscosity Improvers
are temperature sensitive molecules, and progressive in
nature. As the oil temperature
increases, the VIs begin to bond to each other, or swell (depending upon method
used) in a progressive fashion. The
hotter the oil gets, the more effect VI brings to the table. Their purpose is only to help counter the
natural attributes of base stock oils to
thin as they get hotter, witch reduces the ability to protect components. VIs sort of “prop up” the viscosity of base stock oils preventing the “oil” from
getting too thin to be of any value as a boundary layer between moving metal
components. The only real difference
between 10W-30, and 10W-40 oil is the amount of VI needed to attain the higher
40 wt rating at 100 deg C. Recall that
VIs provide absolutely NO lubrication, so if a greater volume of VI additive is
required for the 40 wt. rating, less base stock 10W oil will be in each quart
of “oil”, thereby reducing it’s service life and lubricating abilities. Less base stock is less lubrication
ability. A difference of “25” between the winter and summer ratings is
generally considered the best trade-off between wide temperature viscosity
stabilization, and base stock oil volume (lubricating ability). When the difference is “30” or greater, it is
generally considered that too little base stock remains to be effective over
the long term for lubricating purposes, even though some benefit may be
attained under extreme high ambient loads.
It is better to use oil temperature control strategies, than to use oils
with higher than “25” point spreads in weight for extreme conditions. This is true because also as the VIs get
larger and larger with elevated temps, their resistance to shearing drops, and
overall viscosity suddenly drops dramatically, and oil failure occurs shortly
afterwards, along with the “protected” components. Remember, VIs provide exactly zero
lubrication, they only improve viscosity, or “thickness”.
Another point to consider is that typically, oil
temperatures run approximately 20 deg F. cooler than coolant temperatures under
most operating conditions. The oil is
the last part of the engine to reach full operating temperature, and most often
it’s much lower than the coolant temperature.
Very high duty cycles can dramatically reduce an oil’s service life, if
the oil is maintained at high operating temperatures that are at or above
coolant temperature, for extended periods of time. Higher levels of VIs, associated with higher
“summer” weight fractions, will only make this problem worse. Oil temperature controls must be employed
under these conditions to have acceptable component service life. If adequate temperature controls are
implemented, there is no need to increase the “the oil weight”, buy using 10W,
or 15W multi viscosity oils. If adequate
controls are ineffective, or unavailable, the use of such oils may be advised.
Ratings
are determined by the
American Petrolium Institute (API), to comply with the requirements of the
Society of Automotive Engineers (SAE).
Many years ago, API rated “SA” oils were the state of the art, now they
are unsuitable for use in even the cheapest gasoline powered engines. New oils are far superior to those of even a
few years ago. API ratings are in place
to “certify” that a particular oil meets the requirements of the automotive
design engineers for that weight and brand of oil. You should feel very confident, that an oil
with an API rating, and that is the recommended weight for your vehicle, will
perform to a very high level, and meet or exceed any lubrication
requirements. Also, newer ratings supercede
older ones, and often make them obsolete, in that very way, engine OEM
basically made the old standard 10W-40 SD an obsolete rating when 5W-30 SE was
introduced many years ago. The “later”
the second letter in the alphabet, the “better” the oil. This is due to better additive packages, and
better refining as well. There is also
the issue of “energy conserving” oils, typically in the 5W-30 weight. These “energy conserving” oils can reduce
fuel consumption up to about 7% based upon real-world testing. The API certification will often read
something like API SG/CD. The first
part, SG stands for Spark ignition, G requirement (or certified G), and the
second part CD stands for Compression ignition (diesel), D requirement. The first designation, is the PRIMARY
intended use, and the type for witch the oil was manufactured, the second
designation (if present), is the “will also work for” designation, that does
not imply primary compliance for that type of use, just that the requirements
are met, and very often to the bare minimum.
Synthetic
Oils have numerous benefits, and no known disadvantages,
aside from cost. Some real benefits are:
-pumpability to -60 deg. F.
-zero wax, no or very little, reduction in flow at cold
temperatures (cannot be completely removed from dino oil base stocks).
-need little if any VIs, due to the inherent thermal
properties of “designer” synthetic molecules.
-extremely tolerant of oil temperatures even at or above 300
deg. F., far higher than any conventional oils
-extremely resistant to break-down from long term high
operating conditions
-very stable under high-shear loading
-no need for “straight weight oils”
-oils and synthetic additive packages are completely
uniform, devoid of differing fraction weights
-can suspend eight or more times the chemical and physical
byproducts of combustion as compared to conventional dino oils without
sacrificing lubrication qualities, for deposit in the oil filter.
My experiences
have led me to the following conclusions. For what is probably 98% plus of the vehicles
on the road, 5W-30 oils provide more than adequate component protection for the
entire life of the vehicle, and under all of it’s operating conditions, where
5W-30 oil is recommended. Under SEVERE
ambient and vehicle loading, high (at full temperature for extended periods
such as 5 to 7 out of every 8 hours), or continious duty cycles, 10W-30 oil
would be a better choice for gasoline engines, as the VI is minimized in that
weight oil. For Diesel applications,
15W-40 is typically recommended, and advised for those applications. Under EXTREME cold conditions, and when high
vehicle loading is not a problem, for the Navistar produced “Power Stroke”
engines, 10W-30 oils are often a good second choice.
Under NO circumstances should anyone think that it is OK to
use an oil like 15W-40 Diesel oil (ie:CE/SG) in a gasoline powered engine,
except in dire emergencies. The internal
passage ways, and oil pump components were NEVER intended to pump oils with
that thick. Also the additive packages
for diesel engines, are generally not well compatible with gasoline engines.
If your primary concern is engine wear, you should be most
concerned with start-up wear, and Synthetics offer tremendous benefits there
with their rapid flow under even the most extreme cold temperatures. The synthetics also tolerate short trips
exceptionally well due to their ability to suspend, and handle chemical and physical
contaminants. For these reasons, I use
only synthetic oils in our family vehicles, and follow the manufacturers
recommended intervals (5000 miles) rigorously.
Synthetic oils causing oil leaks, is TOTAL fallacy. It is rumor, and supposition, completely
unsupported by fact. the benefits of
synthetic oils, IMHO, completely outweigh their added costs.